My invention relates to a method of reading digital data on magnetic tape. More specifically, my invention pertains to a reading method for use with magnetic tape having a series of frames each having a predetermined number of data blocks. An electromagnetic transducer or head for reading the tape may be of either the fixed or the rotary type.
As is well known, magnetic tape represents one of the most widely accepted media for the storage of digital data. Usually, a magnetic tape has a recording track constituted of a multiplicity of frames in succession. Each frame has a predetermined number (e.g. sixteen) of data blocks, on which work data is to be stored along with control data, followed by two error checking and correction blocks. When an error occurs in reading any of the data blocks of each frame by the transducer, that data block has so far been reread by rewinding the magnetic tape to an extent necessary for the data block to travel past the transducer again when the tape is subsequently forwarded. Usually, the rereading of any data block has been retried as many as sixteen times if that data block has not been read correctly each time. I will elaborate on this subject in the course of the detailed description of my invention.
The time required for reading each data block during normal operation is of the order of milliseconds. By contrast, in event a read error occurs, each conventional retry has taken as long as one second or so. Thus an inconveniently long time has been necessary for reading each frame when a read error takes place on any one or more of its constituent data blocks. The repetition of the rereading of each data block on which a read error recurs is also objectionable because of the repeated rewinding and forwarding of the magnetic tape and, in consequence, the unavoidable diminution of the useful life of the tape transport.